1. Field of the Invention
The present invention relates to light-sensitive compositions useful as positive-working photoresist compositions having a selected casting solvent mixture. In particular, the present invention relates to light-sensitive compositions useful as positive-working photoresist compositions having an alkali-soluble binder resin and o-quinonediazide photosensitizers dispersed throughout a casting solvent mixture of ethyl lactate and ethyl 3-ethoxy propionate. Furthermore, the present invention relates to the process of coating, imaging and developing with these positive-working photoresist compositions.
2. Description of Related Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally, in these processes, a thin coating or film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to evaporate any casting solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate. In some processes, it is desirable to bake the imaged resist coating before this developing step. This intermediate step is sometimes called post-exposure bake or PEB.
There are two types of photoresist compositions--negative-working and positive-working. Examples of both types of photoresists are well documented in "Introduction to Microlithography", L. F. Thomson, C. G. Willson, and M. J. Bowden, Eds., ACS Symposium Series, 1983.
When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the non-exposed areas of the resist coating and the creation of a negative image in the photoresist coating, and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g. a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics.
In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the following etching step to increase its adhesion to the underlying substrate and its resistance to etching solutions.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases. This latter technique is called plasma etching or dry etching. The etchant solution or plasma gases etch the portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation.
Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface.
The solvents most commonly used in the formulation of commercial positive photoresists are glycol ethers and glycol ether esters such as 2-methoxyethanol (2-ME), 2-ethoxyethanol (2-EE), and their acetates [ethylene glycol monoethyl ether acetate (EGMEA)]. Some commercial photoresists contain a mixture of glycol ethers or ether acetates with xylene, and n-butyl acetate. Evidence has been disclosed, however, that solvents or solvent mixtures containing these glycol ether derivatives have significant toxic effects on the reproductive organs of both male and female test animals at low exposure levels. While no conclusive data yet exists, similar effects may occur with humans. Suitable solvents which possess all of the desired properties, i.e., solubility, wetting, and low toxicity, are, however, difficult to find. As the Environmental Protection Agency (EPA) concluded in a recent investigation on the use of glycol ethers as solvents, "the electronics industry may have severe problems in obtaining feasible substitutes," (CHEMICAL WEEK, p. 7, June 9, 1986).
In response to this evidence, many photoresist manufacturers have introduced or are working on "EGMEA-free safe solvent" photoresist products. The solvent alternatives include "propylene glycol monomethyl ether acetate (PGMEA), ethyl 3-ethoxy propionate (EEP), ethyl lactate, cyclopentanone N-hexanol and bis(2-methoxyethyl) ether (Diglyme)" (SEMICONDUCTOR INTERNATIONAL April 1988 pages 132 and 133).
While ethyl lactate is an effective safe casting solvent with photoresists containing novolak-type binder resins and o-quinonediazide-type photosensitizers, it has been found that when such ethyl lactate-containing photoresists are spin coated onto a relatively large substrate (e.g. 6 inches or greater silicon wafers), the photoresist film resulting after softbake is found to exhibit a deficiency with regard to coating uniformily. Specifically, there exists a variability in film thickness more than conventional EGMEA based resists. Such resulting uneven cast film may cause unacceptable lithographic properties. Accordingly, there is a need for an improved casting solvent over pure ethyl lactate which does not have this uneven spreading problem.